fix(live): refuse billing in pilot mode and explain unparsed documents

- Checkout returns 503 CREDITS_DISABLED while OPT_CREDITS_MODE=off, so a
  customer can never pay for credits that the pipeline will not consume.
- Packs expose purchases_enabled; the portal hides Buy and says billing is off.
- Structuring an unparsed document (e.g. a scanned PDF with no text layer) now
  answers 422 DOCUMENT_NOT_PARSED instead of a generic INVALID_REQUEST.

app/api/0pt/credits/checkout/route.ts

@@ -27,6 +27,15 @@ export async function POST(request: NextRequest) {
   if ('response' in verified) return verified.response;
   const { tenantId } = verified.scope;
 
+  if (process.env.OPT_CREDITS_MODE === 'off') {
+    return NextResponse.json(
+      {
+        error: 'CREDITS_DISABLED',
+        message: 'Billing is not active yet (pilot mode). No payment can be taken.',
+      },
+      { status: 503 },
+    );
+  }
   const book = readCreditPriceBook();
   if (!book) {
     return NextResponse.json(

app/api/0pt/credits/packs/route.ts

@@ -27,6 +27,7 @@ export async function GET(request: NextRequest) {
   return NextResponse.json({
     version: book.version,
     currency: book.currency,
+    purchases_enabled: process.env.OPT_CREDITS_MODE !== 'off',
     packs: book.packs.map((pack) => ({
       id: pack.id,
       credits: pack.credits,

app/api/0pt/documents/structure/route.ts

@@ -163,13 +163,22 @@ export async function POST(request: NextRequest) {
   const dataClass = body.data_class ?? 'low';
   const format = body.format ?? 'json';
 
+  if (pipelineStage !== 'parsed') {
+    return NextResponse.json(
+      {
+        error: 'DOCUMENT_NOT_PARSED',
+        message:
+          'The document has no verified text layer yet (parse_failed or not parsed). OCR is required before structuring.',
+      },
+      { status: 422 },
+    );
+  }
   if (
     !isValidScopeId(projectId) ||
     !isValidScopeId(documentId) ||
     !isValidScopeId(versionId) ||
     typeof checksum !== 'string' ||
     !/^[0-9a-f]{64}$/.test(checksum) ||
-    pipelineStage !== 'parsed' ||
     typeof instruction !== 'string' ||
     !instruction.trim() ||
     instruction.length > 4000 ||